The technical aspects and advantages of sandwich panel construction are well known for use in applications where both structural integrity and light weight are important or required. Sandwich panels with thin-walled cellular cores are frequently called "honeycomb core panels" due to the appearance of the hexagonally-shaped compartments characterized by the assemblage of interconnecting ribbons or strips of material which make up the core structure. The cellular nature of the core can also be achieved by arranging the thin-walled material into compartments of other shapes. These include, but are not limited to, triangular, square, rectangular, octagonal, prismatic, pyramidal, cylindrical and conical shapes, or combinations thereof.
Various materials are used to fabricate such cellular cores, and the coactive face sheets of sandwich panel structures. depending on the structural and functional design requirements of the panel assembly, the materials may include various metals, plastics, fabrics, including paper, or other composite or non-composite materials. Also, depending on the materials selected, adhesives, resins, welding, brazing, diffusion-bonding, or other means of fixing the face sheets to the core may be utilized.
The function of the core in a cellular sandwich panel structure is analogous in a general way to the function of the vertical web of a structural I-beam. Similar to the I-beam web, the primary purpose of the core is to resist and distribute the shear forces throughout the coactive components of the loaded structure, while contributing resistance to crushing, buckling, or warping at any portion on or within the structure. An ideal cellular core structure would allow a sandwich panel assembly to emulate the behavior of a similarly-proportioned, homogeneous solid plate of material under load, at a fraction of the weight. Many cellular core structures occupy less than ten percent of the volume between the enclosing face sheets.
Honeycomb core panels and assemblies are used extensively in the aerospace industry for airframe parts and components such as wing sections, floor panels, seats, and overhead luggage compartments. They are also used in the marine craft industry for assorted wall, ceiling, floor, and cabinet components, in addition to berths, tables, desks, and other furnishings. Honeycomb core panels are being used with increasing frequency in the construction industry for acoustical panels, doors, shelves, and partitions. Unfortunately, the inherent complexity, limited availability, and relatively high cost of manufacturing structural honeycomb core materials impose significant limitation on more widespread use of sandwich panel technology in other panel product applications. Alternate and more practical core configurations for various sandwich panel applications are often sought.
The premise of simplifying the configuration of cellular sandwich panel cores has been the object of numerous patents. Core structures of the prior art have generally been configured on the basis of one or more of the following five primary strategies or concepts:
1. Molding, embossing, vacuum-forming, or otherwise mechanically depressing sheet material into the desired cellular configuration by subjecting the material to relatively large plastic deformations or elongations, effecting substantial reductions in gauge over large portions of the material. This strategy for creating panel cores is exemplified by the following U.S. Pat. Nos.:
3,485,596 dated Dec. 23, 1969 in the name of Jean Alleaume, and entitled Devices Constituting Corrugated Sheet Elements or Plates and Their Various Applications; Shallow, V-shaped ribs or embossments pressed into the sheet material in a pattern constituting an array of adjoining hexagons, with the individual projecting ribs intersecting at specially shaped deformations;
3,525,663 dated Aug. 25, 1970 in the name of Jesse R. Hale, and entitled Anticlastic Cellular Core Structures Having Biaxial Rectilinear Truss Patterns; An array of drawn, circular-capped nodes projecting in opposing directions from the centerline of the sheet, requiring large plastic elongations and substantial reductions in gauge as the elongated portions of material are deep-drawn from the original sheet;
3,785,914 dated Jan. 15, 1974 in the name of Harry A. King, and entitled Structural Material and Means and Method of Making It; Substantially flat-crested, serpentine corrugations pressed into the core sheet material by stretching or molding the core material while in a molten or plastic state;
4,035,536 dated July 12, 1977 in the name of Hadley F. Morrison, and entitled Sandwich Panel Core; A repeating series of alternating hexagonal and triangular embossments stamped, rolled or pressure vacuum-formed into the sheet material, creating a multiplicity of indented compartments;
4,612,225 dated Sept. 16, 1986 in the name of Don E. Graffam et al., and entitled Structural Panel; An array of adjacent, long, open-ended, triangular cells oriented transversely with respect to the centerline of the original sheet material, requiring extensive plastic elongation in the forming process, best suited mainly for thermoplastic materials.
2. Combination of lancing, piercing, or otherwise partially cutting and folding the sheet material into the desired cellular configuration, resulting in openings or penetrations through the core sheet material. This strategy for creating panel cores is exemplified by the following U.S. Pat. Nos.:
3,452,494 dated July 1, 1969 in the name of Hector Thomas Prior, and entitled Multicurved Building Structure; Punched square apertures or openings, with the interconnecting strip portions V-bent into a series of inclined webs;
3,591,351 dated July 6, 1971 in the name of Frederick E. Ullman, and entitled Reticulated Structure and Method of Manufacture; Numerous lancings or piercings of the sheet material, the slitted portions subsequently rotated into a substantially transverse orientation, with respect to the centerline of the original sheet material;
3,673,057 dated June 27, 1972 in the name of Theodore H. Fairbanks, and entitled Cellular Structures; Piercing or slitting the sheet, creating an arrangement of trapezoidal-shaped tabs and subsequently twisting them into an orientation substantially perpendicular to the centerline of the original sheet material;
4,027,058 dated May 31, 1977 in the name of William A. Wootten, and entitled Folded Structural Panel; Creating a series of parallel short slits in the sheet material, and subsequently raising opposing triangular projections out of the slitted regions of the sheet.
3. Truss core, pleated core, or corrugated core structures made with substantially parallel, longitudinally-oriented, rows of relatively simple corrugations. This strategy for creating panel cores is exemplified by the following U.S. Pat. Nos.:
4,035,538 dated July 12, 1977 in the name of Maekawa et al., and entitled Core Block for Plywood and Method and Apparatus for Forming Same; Partially slitting and bend-forming thin plies of sheet material, e.g. wood, into a pleated series of V-shaped ribs, by passing the core material through specialized corrugation rolls;
4,223,053 dated Sept. 16, 1980 in the name of Joseph Brogan, and entitled Truss Core Panels; Assembling a plurality of long, horizontal, triangular tubes together, arranging them side-by-side with their inclined walls adjoining, to form the truss-like core structure;
4,632,862 dated Dec. 30, 1986 in the name of Stephen J. Mullen, and entitled I-Beam Honeycomb Material; Stacking and affixing together multiple layers of sheet material to which has been imparted an alternating sequence of successive U-shaped and N-shaped longitudinal corrugations.
4. Multi-component core structures built-up from a plurality of individual sheet, plate, or strip elements and affixing them together to effect a substantially gridlike bearing surface on one or both of the opposing parallel planes. This strategy for creating panel cores is exemplified by the following U.S. Pat. Nos.:
3,741,859 dated June 26, 1973 in the name of Kurt Wandel, and entitled Reinforced Corrugated Board Member; Inserting a plurality of parallel, vertically-oriented web strips into transverse slots cut into the ribs of a corrugated core structure at regularly spaced intervals to reinforce the structure in its normally weaker direction, perpendicular to the direction of the corrugated ribs;
3,849,237 dated Nov. 19, 1974 in the name of Lev Zetlin, and entitled Structural Member of Sheet Material; Arranging a plurality of triangular-shaped, tetrahedron-shaped, or pyramid-shaped reinforcements into the troughs of a pleated or corrugated core structure to compensate for an otherwise typical lack of support of the face sheets between ridges and valleys of the successive parallel corrugations;
4,573,304 dated Mar. 4, 1986 in the name of David F. Mieyal, and entitled Honeycomb Floor Panel and the Like; Arranging a plurality of vertically oriented, slotted strips together in a grid pattern to form a sandwich panel core structure.
5. Hybrid corrugated sheet structures with secondary ribs or corrugations added to stiffen the inclined sidewalls or flanks of the main corrugations. This strategy for creating panel cores is exemplified by the following U.S. Pat. Nos.:
1,847,216 dated Mar. 1, 1932 in the name of Cecil R. Hubbard, and entitled Packing; A piston rod packing material of soft metal in accordance with the general description above, with the inclined walls of the zigzag folded structure being stiffened by secondary corrugations;
2,896,692 dated July 28, 1959 in the name of Camillo Villoresi, and entitled Method of Making Cushioning Paper; Cushioning or packing medium of corrugated paper such that serpentine or zigzag secondary corrugations are provided;
3,992,162 dated Nov. 16, 1976 in the name of Lucien Victor Gewiss, and entitled Sheet with Alternate Protrusions and Recesses; Alternate protrusions and recesses formed into a sheet material, in a geometry similar to the previous two referenced U.S. Patents, "the walls of each such protrusion and recess being composed exclusively of non-rectangular elementary surfaces;"
4,472,473 dated Sept. 18, 1984 in the name of Randall C. Davis et al., and entitled Curved Cap Corrugated Sheet; A formed sheet structure comprising successive rows of slightly rounded U-shaped corrugations, with the inclined sidewalls being stiffened by secondary corrugations;
4,518,544 dated May 21, 1985 in the name of Thomas P. Carter et al., and entitled Serpentine Film Fill Packing for Evaporative Heat and Mass Exchange; A heat exchanger film packing core structure built up of plural thin sheets having successive rows of serpentine corrugations.